The utilization of electrolytically deposited nickel phosphorus, cobalt phosphorus, and nickel cobalt phosphorus coatings having an amorphous structure has been found to be useful in a wide variety of circumstances. For instance as disclosed in co-pending application Ser. No. 464,101 filed Feb. 4, 1983, a method for producing a fluid jet orifice plate having enhanced utility, by electrolytically coating the substrate metal of the orifice plate with an amorphous nickel phosphorus alloy, is provided. The production of electrical contacts, and other products, utilizing such a coating procedure, also has been recognized. While the plated objects so produced have a number of distinct advantages over like but non-coated articles, to date there has not been a truly significant commercialization of a wide variety of nickel and/or cobalt phosphorus coated articles. This may be due, in part, to the relatively quick destruction of baths used in the plating processes.
According to one conventional procedure, in order to obtain an amorphous nickel and/or cobalt phosphorus coating, the major phosphorus component of the bath is provided by phosphorous acid, with the nickel provided by NiCl.sub.2 and a small amount of NiCO.sub.3, with CoCl.sub.2 replacing the NiCl.sub.2 to various degrees when a cobalt component of the alloy is also desired. Plating can be practiced without any phosphoric acid, but typically a small amount of phosphoric acid (compared to the amount of phosphorous acid) is added to the bath initially in order to facilitate the provision of relatively smooth and bright platings. Such baths are usually operated at as low an anode current density as possible, typically of about 50 amperes per square foot, or less. Upon extended plating utilizing such baths, it has been found that a number of deleterious effects occur in the bath over time. In particular, the platings obtained from the bath degrade in quality over time, in that they are less resistant to corrosion by ferric chloride or concentrated nitric acid. A typical lifetime of the bath before it need be replaced to avoid such quality degradation is about 30-50 ampere-hours per liter. During this lifetime, the cathode efficiency gradually increases from about 40% to about 70%.
According to the present invention it has been found that the major contributor of the deleterious effects on the bath has been the ever increasing concentration of free acid in the bath. A substantial proportion of this free acid is phosphoric acid (H.sub.3 P0.sub.4), which is believed to result from the oxidation of phosphorous acid (H.sub.3 P0.sub.3) at the anode. It has been further found according to the present invention that at low anode current densities this oxidation reaction is substantial, whereas at high anode current densities it is much less substantial, and in fact almost non-existent. Therefore, according to the present invention it has been found that it is possible to provide a bath for plating nickel and/or cobalt phosphorus in amorphous form that shows no significant deleterious effects after 250 ampere hours/liter operation where the anode current density is controlled so as to maintain the phosphoric acid concentration of the bath substantially constant, and so that it does not ever reach a value sufficient to cause deleterious effects. Preferably the phosphoric acid concentration is kept below 0.5 molar. However, it has been found that good plating can be obtained even if the phosphoric acid concentration is up to 4.6 molar, as long as the acid titer is properly controlled. The cathode efficiency of the bath according to the invention retains a value of about 40-50% throughout its life.
While the manifestation of the deleterious effects on the bath is an ever increasing concentration of phosphoric acid, it is believed that the high concentration of phosphoric acid per se is not what results in the deterioration, but rather a condition of overall excessive bath acidity. The desired free acid range in baths according to the invention is so acidic that pH meters are unreliable. Consequently, the free acid concentration is conveniently measured by acid titer. The acid titer is the volume (in milliliters) of deci-normal sodium hydroxide required, when titrating one milliliter of bath, to reach the methyl orange endpoint (which is a pH of about 4.2). The recommended acid titer range is about 9 to 14, representing 0.9 to 1.4 moles/liter of excess acid. The bath is generally maintained at approximately 10 mls. acid titer.
At acid titer below 9, the cathode efficiency decreases, undesirably, to below 30%. In the range of about 9 to 13 cathode efficiency is about 40-60%. Above acid titer 14, cathode efficiency increases to the range of 70-80%, but the corrosion resistance of the plating deteriorates, presumably due to a reduced phosphorus content in the plating. The acid titer is lowered by additions of nickel carbonate and increased by additions of phosphorous acid.
There are alternative ways of measuring the free acid level, such as by measuring the PO.sub.4.sup.-3, HPO.sub.3.sup.-2, Cl.sup.-, and Ni.sup.+2 levels and deriving the acidity. However the acid titer method has proven to be easier in practice.
Preferably, in the practice of one aspect of the method according to the invention, the anode current density is maintained so that it is always greater than about 200 amperes per square foot. At levels significantly below about 200 amperes per square foot the desired control of the phosphoric acid buildup and/or free acid concentration does not occur In fact, anode current densities of at least about 500 amperes per square foot for nickel phosphorus coating baths are preferred. Anode current densities as high as 1250 amperes per square foot are useful, and apparently the upper limits on anode current density are determined by nonelectrochemical constraints, such as I.sup.2 R heating, corrosion of accessory electrical components (such as bus bars) at higher voltages, etc.
According to the present invention, the anode current density is preferably controlled utilizing a particular anode construction vis-a-vis the cathode construction. Typically, the cathode of the bath is provided by the workpiece being coated, such as a fluid jet orifice plate, cookware, cutlery, etc. The cathode-workpiece is immersed in the bath. Disposed adjacent to, but spaced from, the cathode, the anode is immersed in the bath. The precise anode configuration is not critical, as long as the anode's effective surface area is small enough that the current density is in the desired range. Those of ordinary skill in the fixturing art will readily be able to configure anodes suitable for specialized plating. According to a preferred embodiment of the invention the anode comprises a plurality of spaced strips of anode material, and a section of anode may be provided adjacent each major face of the cathode. For example, an anode may be constructed from 125 individually suspended segments of platinum wire, each having a diameter of about 0.010 inches, and each being about 3.23 inches long. It has been found that platinum and rhodium strips (e.g. wires) are more effective over time than other conventional anode materials, such as iridium, gold, palladium, rhenium, and ruthenium. Platinized titanium prevents the oxidation of phosphorous acid, but spalls and in time becomes unusable.
According to the present invention, the production of a wide variety of desirable end products having nickel and/or cobalt phosphorus coatings in amorphous form (that is, having high phosphorus contents, some up to 24 atomic percent phosphorus or more) can be produced in a commercially feasible manner.
It is the primary object of the present invention to provide an improved method and apparatus for the production of nickel and/or cobalt phosphorus electrolytically plated articles utilizing a bath having long life. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.